Modem internal combustion engines have, for many years, been equipped with exhaust gas recirculation mechanisms for routing exhaust gas from their own internal combustion processes back into their intake manifolds, in order to increase efficiency and/or limit the production of undesirable exhaust components, such as nitrogen oxide. For example, introducing exhaust gas into a combustion mixture in an engine's cylinder is known to lower the combustion temperature and, in turn, reduce the formation of nitrogen oxide, as nitrogen oxide forms at elevated temperatures. In order to reduce those elevated temperatures, it is known to cool exhaust gas before introducing it to the combustion mixture. While typical exhaust gas recirculation cooler applications reduce the temperature of exhaust gas from 650° C. to 120° C., the specific cooling requirements for the recirculated gases will often vary according to engine size, type and application.
Typical exhaust gas recirculation coolers are coupled to the internal combustion engine's overall cooling system, and pass exhaust gas through cooling tubes, which are cooled by the engine's radiator coolant. Exhaust gas recirculation coolers have proven to be some of the most complex and historically unreliable pieces of a modern internal combustion engine. These issues have only been exacerbated by the increase in importance as focus has increased on emissions performance and the efficiency of internal combustion engines.
Exhaust gas recirculation coolers must operate under two primary loading mechanisms—thermal fatigue and thermal shock. Thermal fatigue refers to the thermal stresses encountered by exhaust gas recirculation coolers during normal operation. Thermal shock refers to abnormal operating conditions of exhaust gas recirculation coolers, such as the loss of coolant through broken pumps, cooling line failure, etc. Thermal shock is often accompanied by metal expansion of longitudinally oriented exhaust gas recirculation cooler components.
Metal expansion during thermal shock in an exhaust gas recirculation cooler may cause the exhaust gas recirculation cooler to rupture or leak, which, in turn, may negatively impact the overall engine performance. With a cooler leak, coolant may enter the path of the recirculated exhaust gas—back into the intake manifold and, ultimately, the engine cylinder. Any coolant in the engine cylinders impedes the engine's performance and, at certain levels, may completely inhibit the cylinders from firing. Furthermore, if coolant is leaking out of an exhaust gas recirculation cooler, the engine's overall cooling system is affected by that coolant loss. Finally, a leaking exhaust gas recirculation cooler itself may fall to perform its own function—that is to cool the exhaust gases being recirculated to the intake manifold. As set forth above, an elevated temperature of the combustion mixture may lead to undesirable engine and emissions performance.
Two styles of exhaust gas recirculation cooler are known to provide thermal compensation features which accommodate some metal expansion to resist such failure during extreme thermal shock operating conditions. First, it is known for exhaust gas recirculation coolers to employ round, corrugated or convoluted, hollow cooling tubes, which bow and flex to accommodate metal expansion. However, such hollow, round, corrugated tubes have power density limitations, that is, a relatively limited ability to cool exhaust gases passing therethrough, for a given size of tube, as compared to other known constructions for cooler tubes.
Second, it is known to employ floating cores with cooler tubes with relatively higher power density capabilities, such as those with a tube and fin architecture. Cooling tubes with a tube and fin architecture are relatively flattened or oval shaped, with a fin structure bonded inside of a tube, creating an extremely stiff assembly—which expands without compromise under thermal shock conditions. A floating core approach is known to provide a two-piece exhaust manifold, jointly coupled to all of the cooling tubes, which two pieces are movably coupled with an O-ring connection. When the cooling tubes expand, the exhaust manifold components can move relative to each other along the O-ring connection. Such a macro-compensating feature is limited in its effectiveness, however, as exhaust gas recirculation coolers typically do not experience thermal shock on a uniform, macro-scale. Rather, thermal shock conditions typically result in non-uniform expansion of cooling tubes.
Accordingly, an exhaust gas recirculation cooler with relatively high power density and improved thermal shock performance is desirable.